Noise Barriers

[seicer]

Highway Noise Barriers Are a Lie

Highway noise barriers are a pretty common blight on American interstates, there to protect nearby homes and businesses from the motoring sounds of cars and trucks whizzing by. But what if I told you they don't shield noise all that well, and may even amplify it?

In theory, noise barriers seem to make sense, since constructing a giant concrete wall between the offending roadway and the property nearby does block some noise just beyond the wall. But acoustics experts say that the problems start after that.

[jeffandnicole]

I learned a lot of this in some NJDOT public meetings.  Yes, the ones living nearest the highway benefit the most.  But in areas where a road is built or widened thru an existing residential area (often benefiting those downstream), those nearby residents shouldn't have to suffer with the improvements.

The article/opinion statement does do some nice exaggerating work, such as "In many places in this country $92,000 will get you a home or three".  In many places in this country, the $92,000 home isn't right next to a highway. And I'd love to know where I can get 3 for $92,000...that are worth living in, at least!

[myosh_tino]

Out here in California, they're called "Sound Walls".

[hotdogPi]

One of the interesting (and not talked about) side-effects of sound walls is an artifact of the fact that sound waves are affected by gravity.  Most sound walls are intended to stave off complaints from the neighborhoods immediately adjacent to the freeways.  Engineers will take SPL (sound pressure level) meters out to the areas and cobble up some height measurements for the walls based upon the readings.  The wall gets built, and they go out there again and determine the difference in dB (decibels) between "before" and "after" for a given location, normally along the nearest adjacent street paralleling the freeway.  If it's down a certain level (likely 18-24dB or more), they call it a day; if under that figure, they add some height to the wall. 

What walls do to the noise generated by the traffic on the freeway is to amplify that noise directly above the freeway; it's a phenomenon known in the audio industry as "horn-loading"; with only one direction (besides laterally along the freeway itself, which makes little or no difference since the facility itself IS the noise generator) to exhaust itself the sound waves self-amplify vertically.  Eventually those waves are dissipated by gravity and arc outwards from the vertical trajectory -- and eventually return to ground level a certain distance from the freeway/noise generator.  That distance is dependent upon a lot of variable factors: the distance between the sound walls on either side of the freeway, the height of the wall, and even the mean frequency generated by the traffic (diesel engines tend to produce a lower frequency noise spectrum than do gasoline engines). 

Back in the early 90's sound walls were constructed along the CA 85 freeway between I-280 and CA 237; the wall height was about the same as the clearance height of the bridges over the freeway (15-16 feet).  A close friend owned and rented out a house on Wright Street in Sunnyvale, about 6 blocks east of the freeway.  Immediately after the walls were installed, he received complaints from his tenant about increased noise levels -- that sounded suspiciously like freeway noise.  I'm in the speaker design and development business, so he called me for advice; I went over to the rental property and measured the SPL there (you gotta have a portable SPL meter in the speaker business!) as well as on all the parallel streets between Wright and the freeway, as well as 3-4 streets east of there.  While the noise level was relatively low on the street immediately adjacent to the freeway, it increased slightly on the next street over, slightly more on the next street, and ramped up until halfway between the street just west of Wright and Wright Street itself -- right smack on the back patio of my friend's rental house! -- it was about 12-15dB higher than immediately adjacent to the freeway.  East of there the process reversed, each street had slightly lower levels than the next until no significant sound level was detected.  Just my buddy's luck -- his property was the location where the freeway sound waves touched down, diminished by about 10-15dB from the source (we measured that on an overpass) but higher (again about 10-15dB) than the rest of the immediate area.  The noise was centered at about 250 Hz (for you musicians out there, just under middle "C" on a piano, which is 261.6 Hz).

My friend has never been the type to take things lying down, so he complained to Caltrans District 4 about the problem -- but they dismissed it with a "not too bad considering" response, saying there was nothing they could do about it short of capping the freeway, which they were (obviously) not about to consider.  I even wrote a supporting letter suggesting that they place lateral "shelves" along the sound wall to break up the upwards standing waves; there response was that they would take that concept into consideration in future placements, but were not about to retrofit the CA 85 walls in any case. 

He has a new tenant (a divorced college buddy) who tends to play his TV and audio systems louder than normal, so the noise is less of an issue than previously -- but the last time I was on the property, I could still hear the freeway noise distinctly in the back yard.  You can only dissipate sound waves with distance; at close range, you can merely relocate them!

Context can be found by clicking the quote link.

[sparker]

Wow -- and I was about to add a brief(er) summary version of my admittedly long post of about 2 years ago.  Thanks for the reiteration; saves me some time & repetition!

[SectorZ]

"In many places in this country $92,000 will get you a home or three."

At this point I stopped reading. You can write something reconciling gravity with quantum mechanics but if you tell me the Earth is flat in it I'm checking out.

[webny99]

"In many places in this country $92,000 will get you a home or three."
At this point I stopped reading.

That was a bit much. Rochester is known to have fairly cheap real estate, and $92,000 will barely get you one house - and a dated house in an undesirable area of the city, at that.

You want three for that price? Eh, maybe not. (I just happened to stumble across that panorama while looking for cheap homes in the area. There's a foreclosure nearby for 27K).

[jeffandnicole]

As the crow flies, I live a little over 1/4 mile off of I-295, which has sound barriers in the area.  I generally hear two things: Trucks running over the rumble strip, and motorcyclists revving up their engines (a few years back, they were consistently on the highway about 10:50pm every night!).  In the winter, I can hear a little more noise because the trees between me and the highway are bare.  However, the sound is pretty quiet overall.

What bothers me?  PHL, about 3 miles away, especially when the airport is in their Alternate Runway mode, which has planes taking off to the northeast.  Many of them curve right...and right over my house!